In the fabrication of very large scale integrated (VLSI) circuits and ultra large scale integrated (ULSI) circuits, resist materials and lithographic processes which may be used for the high throughput manufacturing of sub-half micron feature sized devices are desirable. Positive working, aqueous base developable, single layer photoresists based on acid amplification, and lithographic processes for their use, are well known. In lithographic processes where a short wavelength exposing light source is used in conjunction with a lens having a high numerical aperture, acid amplified single layer resists can provide sub-half micron resolution. However, the depth-of-focus of exposure tools at such wavelengths is limited. In addition, acid amplified deep UV (DUV) resists often exhibit high sensitivity to environmental contaminants and require special procedures for handling or storing resist materials.
An improvement of the present invention over the single layer resist approach for high resolution patterns entails a bilayer lithographic process which utilizes a relatively thin imaging layer, which typically has a thickness of 0.2 to 0.4 .mu.m, over a 1.0 to 1.5 .mu.m thick planarizing layer. In such a process, the top layer resist materials should satisfy certain fundamental functional requirements. These include high resolution, good image quality, high O.sub.2 etch resistance for low bias after the dry etch transfer of the resist pattern into the underlayer, process reproducibility, and compatibility with clustered manufacturing for high throughput. Existing resist compositions which are not based on photogenerated acid amplification do not have the photospeed necessary for economic thruput with mercury arc DUV steppers. Such compositions also have limited usefulness with excimer laser DUV sources.
Another improvement resulting from the bilayer resist process of the present invention relates to the suppression of optical interference effects. It is generally recognized that patternwise exposure with monochromatic light over reflective surfaces or line edges often gives rise to standing waves within the resist due to interface reflections interfering with the incident light. The scattering and interference effects adversely affect the resist resolution and present a major problem in line width control which is necessary for sub-half micron dimensions. One of the most commonly used approaches to solve the problems associated with light scattering and interface reflections is to add an absorbing dye into the underlayer. However, a dye added to the underlayer is effective in reducing underlayer/substrate interface reflections, but is not effective in reducing imaging resist/underlayer interface reflections.
Thus, in addition to the above requirements for the bilayer overlaying resist, the planarizing underlayer must also have certain specific fundamental optical properties. Two important requirements for the underlayer are: (a) a high optical density at the overlaying resist exposure wavelength; (b) a refractive index which matches the refractive index of the overlaying resist layer, so as to eliminate the line width control problem which is otherwise caused by interface reflections and scattering effects.